102 Rhodora [J UNE 
1. Apices furnished with a ring of tubercles. 
A. Cells broadest at or near base of semicells. 
a. Cells 10-15 times as long as wide 1 " . P. coronatum. 
b. Cells 6-9 times as long as wide . i . ; P. truncatum. 
c. Cells 15-18 times as long as wide. à : . P. Ehrenbergit. 
B. .Cells broadest in middle of semicells. . s . P. subgeorgicum. 
2. Apices usually without tubercles. 
A. Cells of medium size, basal inflation not usually prominent. 
P. Trabecula. 
B. Cells very large, basal inflation prominent. P. maximum. 
II. Cells with a surface pattern of quadrilateral thickenings, ends circular. 
P. verrucosum. 
III. Cells with a few broad constricted areas, sides very sinuate. 
P. constrictum. 
IV. Cells with rings of nodules, end view sinuate-stellate. P. nodosum. 
PLEUROTAENIUM conoNATUM (Bréb.) Rabenh., Flor. Europ. Algar., 
III, 1868, p. 143; Docidium coronatum Bréb., in Ralfs, Brit. Desm., 
1848, p. 217, Pl. XXXV, f. 6; Wolle, Desm. U. S., 1884, p. 49, PI. 
XI, f. 9-10. Cells large, 10-15 times as long as wide, gradually 
attenuated from base to apex, prominent basal inflation, sides undu- 
late, apex truncate with 6-8 blunt tubercles. Length 340-575 y; 
breadth at base 34-63 x; apex 27-40 u.— Me.: Bridgeton! N. H.: 
Noone's Station!; Pudding Pond, North Conway! Mass.: Lake 
Quinsigamond, Worcester (Stone); Plainville! Randolph! Bridge- 
water! 
P. CORONATUM var. FLUCTUATUM West, Jour. Linn. Soc. Bot., Vol. 
XXIX, 1892, p. 118, Pl. XIX, fig. 11. Cells considerably larger 
than in the typical form, 12-14 times as long as broad, sides of semi- 
cells undulate for their entire length. Length 850-900 s; breadth 
at base 65-72 u; apex 46-50 & — N. H.: Intervale! Mass.: Lake 
Watuppa, Fall River! This is one of the largest of our desmids and 
is easily visible without a lens. It is not common. 
P. coRoNATUM var. NODULOSUM (Bréb.) West, Jour. Linn. Soc. 
Bot., Vol. XXIX, 1892, p. 119.  Docidium nodulosum Bréb. in Ralfs, 
Brit. Desm., 1848, p. 155, Pl. XXVI, fig. 1. Semicells with the basal 
inflation and apical tubercles much reduced. Length 560 u; breadth 
at base 65 u; apex 46 j.— Mass.: Amherst (W. West); Salem (Bailey). 
R. I.: Wainskut pond, North Providence (Bailey). I have not found 
this variety myself in New England although it has several times been 
reported by others. The measurements are given from an Ohio speci- 
men which was typical. 
